drm/i915: Fix pipe CSC post offset calculation

We were miscalculating the pipe CSC post offset for the full->limited
range conversion. The resulting post offset was double what it was
supposed to be, which caused blacks to come out grey when using
limited range output on HSW+.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=71769
Cc: stable@vger.kernel.org
Tested-by: NLauri Mylläri <lauri.myllari@gmail.com>
Signed-off-by: NVille Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_display.c

@@ -5815,7 +5815,7 @@ static void intel_set_pipe_csc(struct drm_crtc *crtc)
 		uint16_t postoff = 0;
 
 		if (intel_crtc->config.limited_color_range)
-			postoff = (16 * (1 << 13) / 255) & 0x1fff;
+			postoff = (16 * (1 << 12) / 255) & 0x1fff;
 
 		I915_WRITE(PIPE_CSC_POSTOFF_HI(pipe), postoff);
 		I915_WRITE(PIPE_CSC_POSTOFF_ME(pipe), postoff);